D.14.1 Execution Time Timers
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This subclause describes a language-defined package that provides a facility
for calling a handler when a task has used a defined amount of CPU time.
Static Semantics
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The following language-defined library package exists:
with System;
package Ada.Execution_Time.Timers
is
type Timer (T :
not null access constant
Ada.Task_Identification.Task_Id)
is
tagged limited private;
type Timer_Handler
is
access protected procedure (TM :
in out Timer);
Min_Handler_Ceiling :
constant System.Any_Priority :=
implementation-defined;
procedure Set_Handler (TM :
in out Timer;
In_Time :
in Time_Span;
Handler :
in Timer_Handler);
procedure Set_Handler (TM :
in out Timer;
At_Time :
in CPU_Time;
Handler :
in Timer_Handler);
function Current_Handler (TM : Timer)
return Timer_Handler;
procedure Cancel_Handler (TM :
in out Timer;
Cancelled :
out Boolean);
function Time_Remaining (TM : Timer)
return Time_Span;
Timer_Resource_Error :
exception;
private
... -- not specified by the language
end Ada.Execution_Time.Timers;
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The type Timer represents an execution-time event for a single task and
is capable of detecting execution-time overruns. The access discriminant
T identifies the task concerned. The type Timer needs finalization (see
7.6).
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An object of type Timer is said to be
set if it is associated
with a nonnull value of type Timer_Handler and
cleared otherwise.
All Timer objects are initially cleared.
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The type Timer_Handler identifies a protected procedure to be executed
by the implementation when the timer expires. Such a protected procedure
is called a
handler.
Discussion: Type Timer is tagged. This
makes it possible to share a handler between several events. In simple
cases, 'Access can be used to compare the parameter with a specific timer
object (this works because a tagged type is a by-reference type). In
more complex cases, a type extension of type Timer can be declared; a
double type conversion can be used to access the extension data. An example
of how this can be done can be found for the similar type Timing_Event,
see
D.15.
Dynamic Semantics
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When a Timer object is created, or upon the first call of a Set_Handler
procedure with the timer as parameter, the resources required to operate
an execution-time timer based on the associated execution-time clock
are allocated and initialized. If this operation would exceed the available
resources, Timer_Resource_Error is raised.
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The procedures Set_Handler associate the handler Handler with the timer
TM: if Handler is
null, the timer is cleared; otherwise, it is
set. The first procedure Set_Handler loads the timer TM with an interval
specified by the Time_Span parameter. In this mode, the timer TM
expires
when the execution time of the task identified by TM.T.
all has
increased by In_Time; if In_Time is less than or equal to zero, the timer
expires immediately. The second procedure Set_Handler loads the timer
TM with the absolute value specified by At_Time. In this mode, the timer
TM expires when the execution time of the task identified by TM.T.
all
reaches At_Time; if the value of At_Time has already been reached when
Set_Handler is called, the timer expires immediately.
Implementation Note: Since an access-to-constant
can designate a variable, the Task_Id value designated by the discriminant
of a Timer object can be changed after the object is created. Thus, an
implementation cannot use the value of the Task_Id other than where this
Standard specifies. For instance, the Task_Id should be read when the
timer is set, but it should not be used when the timer expires (as it
may designate a different task at that point).
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A call of a procedure Set_Handler for a timer that is already set replaces
the handler and the (absolute or relative) execution time; if Handler
is not
null, the timer remains set.
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When a timer expires, the associated handler is executed, passing the
timer as parameter. The initial action of the execution of the handler
is to clear the event.
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The function Current_Handler returns the handler associated with the
timer TM if that timer is set; otherwise, it returns
null.
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The procedure Cancel_Handler clears the timer if it is set. Cancelled
is assigned True if the timer was set prior to it being cleared; otherwise,
it is assigned False.
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The function Time_Remaining returns the execution time interval that
remains until the timer TM would expire, if that timer is set; otherwise,
it returns Time_Span_Zero.
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The constant Min_Handler_Ceiling is the minimum ceiling priority required
for a protected object with a handler to ensure that no ceiling violation
will occur when that handler is invoked.
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As part of the finalization of an object of type Timer, the timer is
cleared.
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For all the subprograms defined in this package, Tasking_Error is raised
if the task identified by TM.T.
all has terminated, and Program_Error
is raised if the value of TM.T.
all is Task_Identification.Null_Task_Id.
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An exception propagated from a handler invoked as part of the expiration
of a timer has no effect.
Erroneous Execution
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For a call of any of the subprograms defined in this
package, if the task identified by TM.T.
all no longer exists,
the execution of the program is erroneous.
Implementation Requirements
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For a given Timer object, the implementation shall perform the operations
declared in this package atomically with respect to any of these operations
on the same Timer object. The replacement of a handler by a call of Set_Handler
shall be performed atomically with respect to the execution of the handler.
Reason: This prevents various race conditions.
In particular it ensures that if an event occurs when Set_Handler is
changing the handler then either the new or old handler is executed in
response to the appropriate event. It is never possible for a new handler
to be executed in response to an old event
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When an object of type Timer is finalized, the system resources used
by the timer shall be deallocated.
Implementation Permissions
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Implementations may limit the number of timers that can be defined for
each task. If this limit is exceeded, then Timer_Resource_Error is raised.
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A Timer_Handler can be associated with several Timer objects.
Extensions to Ada 95
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
